BACKGROUND AND OBJECTIVES: Pediatric low
flow anesthesia requires adequate equipment which, when available, is extremely
expensive, thus seldom used. This study aimed at evaluating low flow anesthesia
in rabbits using a closed rebreathing circuit in a new pediatric pressure controlled
ventilator for anesthesia.METHODS: Ten rabbits were randomly assigned to two groups. Group I individuals
were ventilated with the airway pressure limit set to 15 cmH2O, while
in group II the setting was 20 cm H2O. Anesthesia was induced with
muscular xylazine (10 mg.kg-1) and ketamine (25 mg.kg-1),
followed by maintenance with isoflurane after tracheal intubation. After 20
minutes, 0.1 mg.kg-1 intravenous pancuronium was administered and
controlled ventilation was established. Ventilator parameters were: RR - 30
mpm, I:E ratio 1:2.5 and inspiratory time 0.6 sec, in addition to plateau pressures.
Fresh gas flow was 300 ml.min-1 (total). Parameters were collected
every 20 minutes for one hour and data were submitted to analysis of variance
for repeated measures (p < 0.05).RESULTS: Re-inhaled CO2 decreased significantly in group II
from an initial value of 15 mmHg during spontaneous ventilation to a mean value
of 2.4 mmHg during controlled ventilation. In group I, the drop was from 19.2
mmHg (initial) to 3.6 mmHg. Comparing both groups, significant differences were
observed in venous pH, PaCO2, PvO2 and a slight difference
between MBP and DBP. The 15 cmH2O group showed important respiratory
acidosis, while the 20 cmH2O had normal pH and PaCO2 values.
Since expired volume values were similar in both groups, such differences in
pH and blood gases observed could be related to low pH levels seen in group
I. Each animal consumed a mean value of 2 ml isoflurane during the 120 minutes
of the study.CONCLUSIONS: With proper equipment, it is possible to use low flow anesthesia
with pressure controlled ventilation and closed system in very low weight patients.

There has been an increasing interest in the
use of closed circuits in children in the last years. Such circuits associated
to high precision ventilators allow for the use of lower fresh gas flows, better
moisture and heat preservation and clearly less anesthetic consumption and operating
room pollution1. An additional benefit is the possibility of using
different ventilatory modalities and resources, such as positive end expiratory
pressure (PEEP) available in such equipment, which are certainly needed for
high risk patients. However, in our country, pediatric anesthesia has been a
challenge for the anesthesiologist due to the lack of adequate equipment. In
many centers, non-rebreathing circuits such as Bains or Mapleson D systems
are still widely use in pediatrics with manually controlled ventilation. These
circuits use high fresh gas flows leading to high anesthetic consumption and
environmental pollution. Another great disadvantage of such practice is the
lack of accurate parameters monitoring, such as expired volume and airway pressure,
among others.

A major drawback for the use of adult ventilators
and anesthesia closed circuits, which could also be used for low weight patients,
is their high cost. The possibility of low flow anesthesia in children would
be another great advantage of this new equipment available in the market.

This study aimed at evaluating a closed circuit
anesthesia machine using a pressure controlled time-cycled ventilatory mode
with low fresh gas flow. Small animals (rabbits) were used to check the efficacy
of this ventilator during anesthesia.

METHODS

Ten male and female rabbits weighing 3.5 to 5
kg were used. Anesthesia was induced with muscular xylazine (10 mg.kg-1)
and ketamine (25 mg.kg-1), followed by isoflurane under mask. Tracheal
intubation was performed with Magills cuffed tubes (2.5 to 3.0 of internal
diameter). Anesthesia was maintained with isoflurane in 100% oxygen. Animals
remained under spontaneous ventilation for 20 minutes until stabilization of
the anesthesia machine parameters. Marginal artery and ear vein were catheterized
for blood pressure monitoring as well as fluid (2 ml.kg.h-1 lactated
Ringers) and drugs administration, respectively. Pancuronium (0.1 mg.kg-1)
was then administered and time-cycled pressure controlled ventilation was installed
for 60 minutes. Ventilator parameters were: respiratory rate of 30 movements
per minute, I:E ratio of 1:2.5 and inspiratory time of 0.6 seconds.

Fresh gas flow was 300 ml.min-1 delivered
by a non pressure gauged flowmeter. Animals were then randomly distributed in
two groups of five animals each. The ventilatory peak pressure was limited to
15 cmH2O in Group I (G15) and 20 cmH2O in Group II (G20).
Ventilator gas flow was adjusted to generate a square pressure curve, compatible
with the desired modality.

Using a ventilation monitor, inspired and expired
volume, minute volume, peak, mean and plateau pressures, as well as pulmonary
compliance were measured too. This monitor has assured respiratory modality
through the visualization of pressure curves. To monitor anesthesia, inspired
and expired isoflurane concentration, electrocardiogram and blood pressure were
continuously evaluated.

A pneumatic time-cycled system with inspiratory
and expiratory phase control was used for this study with the following adjustable
controls: flow, inspiratory time, expiratory time, maximum pressure and PEEP,
in addition to an alarm detecting disconnection or lack of cycle.

3. One subset made up of a combination of valves
which allow the device to operate in controlled (manual or mechanical) or
spontaneous ventilation, and;

4. One CO2 absorber reservoir.

During the expiratory phase, a sub-atmospheric
pressure acts between the internal reservoir bag (7) and the dome (8), allowing
it to be filled with the gaseous mixture after going through the unidirectional
valve (1). Another unidirectional valve (2) prevents the expired volume to return
to the bag (7).

During the inspiratory phase, a gas volume (generated
by the flow control during inspiratory time) compresses the internal reservoir
bag (7). The speed of this compression is a function of the flow set, which
should mimic a manual compression.

The unidirectional valve (1) closes and the
valve (2) opens, allowing the bags content (7) to reach the patient.

An expiratory valve (3) holds the tidal volume
until the previously set airway pressure (maximum inspiratory pressure) is reached.

From this point on, the volume in excess escapes
by opening and overcoming the unidirectional valve (4), and may go:

a) to the external reservoir bag (6);

b) to the atmosphere or anti-pollution system
through the pop-off valve (5), if the external reservoir bag (6) is full;

c) directly to the atmosphere if the system
is open.

An additional gas flow (AGF) placed between valves
(1) and (2) allows the second valve to be kept open, regardless of AGF and provided
the internal reservoir bag (7) is full. This allows the small patient to ventilate
without resistance. During the expiratory phase, the internal reservoir bag
(7) is filled with the gaseous mixture coming from the external reservoir bag
(6), as well as from the volume expired by the patient (after going through
the CO2 absorber). If the system in use has no CO2 absorber,
atmospheric air is then aspirated.

The valve (9) connects the external wall of the
internal reservoir bag (7) to the atmosphere, and this happens with the equipment
off (spontaneous and manually controlled ventilation). This valve also acts
as a safety valve for the Bag in a Bottle system by preventing the
pressure at this point to go beyond 80 cm H2O.

The expiratory branch was chosen to accurately
measure mouth pressure, what was done before the valve (3). This measurement
doesnt reflect the resistive pressure of flow going through the inspiratory
way.

The configuration may be converted to a non CO2
absorbing system when one or both corrugated tubes (10 and 11), which connect
the ventilator to the absorbing canister, are disconnected. Such procedure may
be performed when a faster emergence is desired.

The equipment allows intermittent mandatory ventilation,
provided that expiratory time be increased.

Results obtained were analyzed by a computer
system (INSTAT), being data submitted to ANOVA analysis of variance and Students
t test to compare means of both groups. Significance level of 5% was established.

RESULTS

Results are shown in table
I, table II
and table III.
There were no statistically significant differences between both experimental
groups as to control and spontaneous ventilation moments.

During spontaneous ventilation, high inspired
and expired CO2 values were observed in both groups. With controlled
ventilation, there has been a ventilation increment, confirmed by PETCO2
normalization. Inspired CO2 decreased and remained in values compatible
with an adequate ventilation.

When comparing both experimental groups, the
only significant differences were seen with PaCO2, PvO2,
PvCO2 and venous pH. The group ventilated with 20 cmH2O
had lower PaCO2 (figure
2) and higher PvO2 and pH values. As to heart rate and blood
pressure, there was no significant difference between groups or between evaluations
performed before and after controlled ventilation.

During controlled ventilation, isoflurane consumption
was 2 ml per animal.

After 90 minutes of anesthesia, isoflurane was
withdrawn and animals were extubated uneventfully.

DISCUSSION

The lack of adequate and reliable anesthesia
ventilation equipment has historically limited the use of pediatric low flow
techniques in our country. This concept is also applied to veterinary anesthesia
in small animals as well to latu sensu pediatric anesthesia. There
are several definitions to low flow anesthesia. One of them refers to the use
of 0.5 to 1 L.min-1 of fresh gas flow, while minimum flow implies
the use of values below 0.5 L.min-11. There is no distinction
regarding body weight. Recently, Tobin et al.2 have evaluated the
efficacy of a closed system with adult bellows to provide minute volume to pediatric
test lung. Authors have shown that when an adult closed system is used in children,
ventilation depends basically on respiratory rate, inspiratory peak pressure
and lung compliance rather than the ventilation mode, suggesting that it is
feasible to use an adult closed system for pediatric patients. Igarashi et al.3
have also shown that the use of a closed low flow anesthesia system (600 ml.min-1)
is very feasible in children.

In our study, the pressure controlled closed
system allowed adequate ventilation and oxygenation of low weight individuals
without the inconvenience of high fresh gas flows and with low consumption of
inhalational anesthetics.

The ventilation modality provided high and adequate
expired volumes (EV), even with narrow tracheal tubes and constant respiratory
rate of 30 movements per minute. Tobin et al.2, in an experimental
study, have observed that in a normal compliance lung, tube diameter, respiratory
rate and PEEP values are limiting factors to obtain adequate expired volumes.
In our study, however, despite PEEP values variations with the use of a 3.5
tube and a respiratory rate of 30 movements per minute, adequate EV volumes
were reached, confirmed by ventilation parameters seen throughout the study.
In ventilation, especially pediatric one, resistive forces may impair alveolar
ventilation, especially if associated to sudden compliance variations. Animals
ventilated with 20 cm H2O have clearly shown such fact.

By concept, lung ventilation in closed system
anesthesia depends on the driving force generated by the ventilator,
and the hole of gas flow is to supply the anesthetic agent and oxygen to tissue
needs.

According to current concepts of low fresh gas
flow anesthesia, our study was performed with minimum flow anesthesia 1,4,5,
since we used 300 ml.min-1 of oxygen flow. To date, in spite of initial
calculations in which the choice of gas flow used in low flow anesthesia is
a function of oxygen consumption per kilogram, such definition does not take
into account patients weight. Igarashi et al.3 have used 600
ml.min-1 of fresh gases in pediatric patients with mean weight of
16.4 kg to study low flow anesthesia with sevoflurane. Perkins et al.6
have used 800 ml.min-1 in children with mean weight of 15 kg.

There are several advantages in the use of low
fresh gas flows: decreased operating room pollution, better moisture and temperature
maintenance of inspired gases and lower anesthetic consumption1,4,5.

Baxter 5 has reported a 25% anesthesia
costs decrease with fresh gas flow reduction to at 1 L.min-1. Perkins
et al. 6 have observed an approximately 58% decrease in isoflurane
consumption while Igarashi et al. 3 have noticed that sevoflurane
consumption was 1/7 of the total used during anesthesias with conventional oxygen
flows (6 L.min-1). In our study, isoflurane consumption was 2 ml
for one-hour anesthesia.

According to Eger 7, the increasing
use of the new generation inhalational anesthetics, sevoflurane and desflurane,
shall increase even more the use of low flow techniques. For having a low solubility,
such anesthetics are absorbed in smaller amounts. Also, due to their lower potency,
high partial pressures in the anesthesia circuit are needed. With high fresh
gas flows, most exhaled gas is wasted, and a great amount of anesthetics has
to be vaporized and delivered into the anesthesia circuit to re-establish high
partial pressures needed to maintain an adequate anesthesia depth 1,8.
For all these reasons, the only way to optimize the use of the new agents is
to use a lower fresh gas flow.

In most studies dealing with low flow anesthesia
in children, anesthesia machines are highly sophisticated, what must be related
to greater hospital investments. Our study has evaluated a new equipment which
has shown to be very safe and easy to use. Spending less with equipment and
using less anesthetic drugs, it is possible to invest in the purchase of ventilation
and anesthetic gases monitoring devices, mandatory for low fresh gas flow anesthesia.

In conclusion, in our study, the use of the ventilator
(VLL-5000) coupled to conventional flowmeters and vaporizers was efficient and
safe in gaseous exchange, maintaining an adequate anesthesia depth in the species
under investigation and allowing the use of low gas flows.

ACKNOWLEDGEMENTS

The authors thank Dr. Humberto do Val for lending
the equipment for the experiment, as well as for supplying technical information
on it.